Railing post should have been a prerequisite thought in the process of building any deck. As far as attaching to Aztec or trex product decking it is not sufficiently structural to withstand lateral forces needed for adequate attachment. Which is why you can't find any attachment method, as it isn't recommended.
Sounds like you made your own decision on your approach to the problem. Unfortunately if someone gets hurt because the railing "let go" who would be at fault?
My suggestion is to reinstall new posts either through the decking and attached directly to the underlying joist or surface mounted outside the decking perimeter with carriage bolts(zinc coated). The Aztec decking "will" shrink with time and loosen any type of nails attached, so think about that.
This could be a DIY project, but it's pretty big. Anytime you start messing with support columns you can get yourself into a big project. But, that said, this is most likely doable.
You need to get the bottom of the post off the ground with some sort of concrete footing and replace the bottom (or all) of the post.
Basically, if you hired a contractor, he'd prop up the roof using doubled 2x8's on each side of the post. Then he'd either replace the whole post or cut off the post about 2 feet from the ground. He'd probably (based on code) also dig a hole and pour a concrete footing and then bolt the new post base back in. The size and depth of the footing can vary by local regs and the contractor's preference.
If it were me, I'd prop up the porch, take out a post, dig a hole (2 foot+, unless you are concerned about frost heave in which case 4 feet+), put in a sono tube up to about a foot above ground, fill it with concrete and put a new post in.
Based on local code, this would probably require a permit (but depends on where you are and the nerve of your contractor - personally, I'd do this myself without a permit...)
You reuse the metal bracket by embedding in the top of the new footing (sand it and paint it with rust proof paint), but you might not need it. Home Depot has lots of various bracket to attach vertical columns to footings. Also, google images is your friend on how this can look.
Best Answer
As you have access to the bottom of the post, consider to use a wood bit and drill a series of large diameter (1") as close to the perimeter as possible without striking the metal. Go as deep as the metal bracket, which will weaken the wall of the post.
Obviously if you intend to re-use the post, it will be less well suited at the full length. As such, you can also cut off the post above the bracket and perform the drilling all the way through, creating an even weaker bond with the bracket.
Inertial is important in this type of removal. If you can cut a hole in a sturdy plank that is exactly the size of the post, but not as large as the bracket, brace the plank and drive the post through the hole with it's own weight. When the bracket reaches the hole, it will budge. It may take a few shots to break it free.